A new class of Ig which has recently been identified on murine lymphocytes has been tentatively described as IgD because of similarities with its human counterpart. Preliminary studies indicate that the expression of this Ig is thymic independent and also perhaps antigen independent. Although it appears after IgM during ontogeny, it may be the major cell surface Ig on lymphocytes from adult mice. For this reason the study of IgD is considered essential in understanding B cell differentiation and function. The objectives of this proposal are to (1) definitively identify this Ig as IgD by the production of a cross- reacting antiserum (2) determine if development of IgD is antigen independent by using antigen-free mice, (3) show that IgD receptor has the same antigenic specificity as IgM by using idiotypic antibody to precipitate both classes from lysates of iodinated immune cells, (4) study the relationship of the presence of IgD to the homing properties of lymphoid cells by using radioiodinated cells in transfer experiments, (5) establish the role of the spleen as a site for differentiation of cells from IgM to IgD by using splenectomized and asplenic mice, (6) study IgD-biosynthesis and shedding in resting and stimulated cells using previously described techniques, and (7) establish whether a switch from IgM to IgD occurs in differentiating B lymphocytes by using irradiated bone-marrow reconstituted animals.